1. Field of the Invention
Aspects of the present invention are directed to the measurement and control of the rate of flow of a fluid, and more particularly to a thermal mass flow sensor that may be included in a thermal mass flow meter or a thermal mass flow controller, and that is associated with an inclination sensor.
2. Discussion of the Related Art
Thermal mass flow meters and thermal mass flow controllers are well known in the art. It is also well known in the art that the orientation of a thermal mass flow meter (or a thermal mass flow controller) may affect its performance. For example, due to a phenomenon termed the “recirculation effect,” the accuracy of a thermal mass flow meter (or thermal mass flow controller) may vary considerably from one orientation to another. This recirculation effect is now briefly described with respect to a conventional thermal mass flow controller depicted in FIG. 1.
As illustrated in FIG. 1, thermal mass flow controller 100 includes a base 110 having a fluid inlet 120 and a fluid outlet 130, a thermal mass flow meter 140, a valve 150, and some associated control electronics 160. In the illustrated example, the thermal mass flow meter 140 includes a pressure dropping bypass 142 through which a majority of fluid flows, and a thermal flow sensor 146 through which a smaller portion of the fluid flows. The thermal flow sensor 146 includes a sensor inlet portion 146A, a sensor outlet portion 146B, and a sensor measuring portion 146C about which two resistive coils or windings 146D, 146E are disposed.
In operation, electrical current is provided to the two resistive windings 146D, 146E which are in thermal contact with the sensor measuring portion 146C. The heat generated by the resistive windings 146D, 146E is used to heat the fluid flowing therein to a temperature that is above the temperature of the fluid flowing through the bypass 142. As known to those skilled in the art, the rate of flow of fluid in the flow sensor 146, which is proportional to the rate of flow of fluid through the mass flow controller 100, may be determined in a number of different ways, such as, by a difference in the resistance of the resistive windings, by a difference in the amount of energy provided to each resistive winding to maintain each winding at a particular temperature or at a particular temperature above ambient temperature, etc. Examples of the ways in which the flow rate of a fluid in a thermal mass flow meter may be determined are described, for example, in commonly owned U.S. Pat. No. 6,845,659 B2, which is incorporated by reference herein.
When the valve 150 is in a closed position and the thermal mass flow controller 100 is disposed in the position shown in FIG. 1 with the sensor measuring portion 146C oriented horizontally (i.e., with the sensor measuring portion oriented parallel to the X axis shown in FIG. 1), the fluid in the sensor measuring portion is heated to a temperature above that of the fluid in the bypass 142. Because of this heating, the density of the fluid in the sensor measuring portion is less than the density of the fluid in the cooler bypass 142. Thus, in the orientation shown, and with the valve in a closed position, a density gradient will develop and be maintained by the force of gravity, with the heavier and more dense fluid in the bypass and with the lighter (more buoyant) and less dense fluid in the sensor measuring portion.
If, instead of being disposed in the orientation depicted in FIG. 1, the thermal mass flow controller 100 is instead disposed in a different orientation, for example, in the manner shown in FIG. 2 (i.e., with the sensor measuring portion 146C being oriented parallel to the Y axis), convective forces due to differences in temperature between the fluid in the sensor measuring portion and that in the bypass 142 and the force of gravity may give rise to a recirculation of fluid that can affect the accuracy by which flow rate is determined.
For example, with the valve 150 in a closed position and the thermal mass flow controller 100 disposed in the position shown in FIG. 2, the fluid in the sensor measuring portion 146C will again be heated to a temperature above that of the fluid in the bypass 142. As a result of this heating, the warmer and less dense fluid in the sensor measuring portion rises and flows out of the sensor outlet 146B where it contacts the bypass and cools, while the cooler fluid in the bypass is in turn pulled into the sensor inlet 146A and warmed by the resistive windings 146D, 146E. Due to a combination of convective forces and the force of gravity, a continuous circulation loop is created which will be detected as a flow of fluid by the control electronics associated with the mass flow controller, despite the fact that the valve is closed. It should be appreciated that depending upon the orientation of the mass flow controller, this false flow signal may be detected as a positive (e.g., increased) flow of fluid (for example, in the orientation shown in FIG. 2) or as a negative (e.g., decreased) flow of fluid. Further, although the effect of such recirculation is most noticeable when the valve 150 is in a closed position, it should be appreciated that this phenomenon will also affect the accuracy by which flow rate is determined when the valve is open, with its impact being more significant at lower flow rates and with more dense fluids.
A number of different mass flow meters and/or mass flow controller designs have attempted to mitigate the effects of recirculation, such as, for example, those described in commonly owned U.S. Pat. Nos. 5,279,154 and 6,044,701. Although the devices disclosed in the afore-mentioned U.S. patents do reduce the effects of recirculation, that reduction is generally dependent upon installing and using the mass flow meter and/or mass flow controller in the specific orientation in which it was designed to be used.